Weakly Coupled 19-Core 5-Mode-Group Ring Core Fiber for Dense Space-Division Multiplexing

We propose a 19-core 5-mode-group ring-core fiber capable of supporting up to 380 spatial channels. Each core can support 5 orbital angular momentum (OAM) mode groups (MGs) with topological charge (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${\mathit{l}}$</tex-math></inline-formula>) ranging from 1 to 5, all of which have a radial order of 1. In each MG, the number of degenerate modes is 4, which indicates that the scale of multiple-input multiple-output digital signal processing (MIMO-DSP) can be fixed at 4 × 4 at the receiver side. To maintain weak inter-MG coupling, our fiber design focuses on achieving a large effective index difference (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\Delta n_{eff}$</tex-math></inline-formula>) between OAM MGs and minimizing the mode field overlap. The fiber consists of 19 trench-assisted ring cores arranged in a compact cladding region, with a large core pitch to reduce inter-core crosstalk. We conduct thorough optical and communication performance evaluations on the fabricated fiber. Experimental results show that the crosstalk between adjacent lower-order MGs (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\vert {\mathit{l}} \vert$</tex-math></inline-formula> <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$=$</tex-math></inline-formula> 1, 2, 3) is below <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$-$</tex-math></inline-formula> 30 dB/km, and that between higher-order MGs (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\vert {\mathit{l}} \vert$</tex-math></inline-formula> <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$=$</tex-math></inline-formula> 3, 4, 5) is below <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$-$</tex-math></inline-formula> 27 dB/km. We also demonstrate the transmission capability of this fiber by multiplexing five OAM MGs across three cores over a 3.5 km span in the C band. Each channel transmits a 24-Gbaud dual-polarization quadrature phase shift keying (QPSK) signal, with signal impairments compensated using a 4 × 4 MIMO-DSP algorithm. All signals achieve a bit error rate (BER) below the 20% soft-decision forward error correction (FEC) threshold. Given its low inter-channel crosstalk and robust communication performance, this fiber holds good promise for supporting ultra-high-capacity optical transmission systems.